1. Field of the Invention
The invention relates to a hydraulic pressure control apparatus for a vehicle power transmission system.
2. Description of the Related Art
There is a hydraulic pressure control apparatus for a vehicle power transmission system, which includes a first regulation unit (primary regulator valve) and a second regulation unit (secondary regulator valve) (refer to, for example, Japanese Patent Application Publication No. 2007-78011 (JP-A-2007-78011)). The primary regulator valve regulates the hydraulic pressure generated by an oil pump (discharge pressure) to form a line pressure that is used as the original pressure (control original pressure) for the hydraulic pressure that is supplied to each element. The secondary regulator valve is provided downstream of the primary regulator valve, and regulates the hydraulic pressure discharged from the primary regulator valve to form a secondary pressure. The secondary pressure formed by the secondary regulator valve is applied to, for example, a control valve that is actuated to apply or release a lockup clutch of a fluid power transmission unit (torque converter) provided in the vehicle power transmission system.
In an existing hydraulic pressure control apparatus, a secondary regulator valve is configured in such a manner that only one pilot pressure is introduced into the secondary regulator valve. That is, the secondary regulator valve is configured to form the secondary pressure using only one pilot pressure. Therefore, if the pilot pressure changes, the secondary pressure also changes. As a result, the durability of a torque converter may be reduced due to an increase in the secondary pressure, or the degree of application of a lockup clutch may be reduced due to a decrease in the secondary pressure.